A central office or exchange of a PCM/TDM telecommunication system generally comprises a processor which monitors the activities of the subscriber lines and allocates to each active subscriber, communicating therewith either directly or through a remote transit exchange, both an incoming and an outgoing PCM channel; a switching network then serves to transfer PCM-coded messages from one subscriber, arriving at the central office via the incoming channel allocated to that line, to the outgoing channel allocated to another subscriber called by (or calling) the first one, with interim storage of the message bits between time slots assigned to the two intercommunicating channels. The switching network must therefore receive from the processor, during a recurrent TDM frame composed of a multiplicity of time slots, the identification codes or addresses of all the allocated channels which are to be operatively coupled together in the course of such a frame.
Thus, an interface unit inserted between the processor and the switching network must register these address codes and feed them to the switching network at the proper instants in order to indicate the correlation among active channels. That information, of course, must be updated from one time slot to the next.
If the number of channels handled by the exchange is large, the interface unit must have a correspondingly high storage capacity; conventional memories capable of being read rapidly enough to supply all the requisite operating instructions to the switching network within the time available therefor, however, have only a limited capacity.